Research On The Issues Of Fetal Magnetic Resonance Safety

Since the magnetic resonance imaging (MRI) advent, with the advantages of high contrast resolution for the soft tissues, arbitrary direction directly slice imaging, no damage, no ionizing radiation and so on, have been widely used in clinical application. The MRI was applied to the clinical examination of the human fetus since from twentieth Century 80’s first attempt and developed gradually. Before magnetic resonance imaging was applied to the fetus, routine screening equipment for diagnosis of fetal was ultrasound (US). Compared with the ultrasound examination, the magnetic resonance imaging had high spatial resolution, but the early magnetic resonance equipment and examination cost was expensive, scanning time was very long, motion artifact was difficult to avoid. After the 90’s, with the development of various magnetic resonance techniques such as sampling technology, coding technology, sequence improvement, imaging time is reduced greatly, the affection of fetal movement on image has been greatly reduced, fetal MRI is gradually applied to examination of fetus.Fetal MRI is widely used in the examination of the central nervous system. It has reliable diagnosis information for ventricular expansion, corpus callosum dysplasia, the posterior fossa lesion (various deformities and posterior fossa lesions), cortical dysplasia, arachnoid cyst, transparent membrane cyst and hemorrhage, calcification, diagnosis is reliable and it has better diagnosis effect than US for fetal heart disease such as cardiac rhabdomyoma, pericardial cyst, IN summary, magnetic resonance imaging has larger field of view, has high soft tissue contrast resolution, is not affected by the condition of mother and amniotic fluid, no radiation, no damage, is suitable for the diagnosis of fetus. It can complement the limitation of the ultrasound, provide more prenatal diagnosis information, the development of fetal magnetic resonance imaging in the domestic and foreign is very rapid.Considering the movement of the fetus was not in control, fetal MRI scans tend to use fast imaging sequences, for example:balanced steady-state free precession sequence (the balance-FFE) and single excitation fast spin echo sequence (SSTSE), these are the two most used sequence. Diffusion weighted (DWI) sequence (for ischemia), fMRI(functional MRI)sequence (for the acoustic reflex), MRS (magnetic resonance spectroscopy, for metabolites of liver, heart, brain) and many other sequences can be used to the fetus.The base of MRI is the principle of nuclear magnetic resonance (NMR). It is no radiation and no ionizing radiation, so far, has not yet found that short time exposure to static magnetic field will cause harm to the growth of fetus. In spite of this, the embryo at the stage of cell differentiation in the first trimester of pregnancy. It is not clear what and how the outside factors will bring damages to the fetus development, for maximum protection of the fetus, the general advice is for fetus within 3 month, not advocating do magnetic resonance imaging. To process fetus more than three months, usually choose field strength no more than 1.5 T magnetic resonance equipment. The reason that do not select a higher field strength magnetic resonance system is not the higher static magnetic field strength will bring obvious harm, but the organization will be warmed by the radio frequency (RF) pulse during magnetic resonance imaging and the acoustic noise which due to the vibration resulted from Lorentz force generated by the high-speed switch of the current in the gradient coil. They may be more prone to the development of the fetus. The higher the field strength, the higher the radio frequency energy and the higher gradient noise. This article mainly aims at studying the absorption of radio frequency energy of the human body when pregnancy and the noise protection for fetal magnetic resonance imaging during high field (3.0 T) magnetic resonance imaging.1 research of radio frequency hotSpecific absorption rate (SAR) refers to the radio frequency absorption energy of the unit object mass (W/KG). It is an important parameters to characterize the radio frequency energy absorbed by the human body. The FDA, IEC, ICNIRP are made corresponding provisions for SAR values.SAR values basic calculation formula is as follows:We can see from the formula that to calculate the SAR values of every bit, the conductivity, density and field intensity of every point are needed. Early research of SAR value simulation calculation are on the assuming of homogeneous field and homogeneous medium model. Because of the uniformity of the electromagnetic field will be affected by human body, and the body tissues and organs of conductivity and density are heterogeneous, therefore, there is great error between the simulation results on the basis of the uniform field and medium hypothesis and real SAR value distribution. In order to get more accurate results, we use the FDTD method to establish the birdcage coil simulation model of common magnetic resonance system and three dimensional electromagnetic model of female pregnancy pelvic cavity for SAR value simulation calculation.Basic principle of finite difference time domain method is:on the basis of Maxwell differential equations, discretization sampling was performed in the space domain and the time domain alternately. Finally, a center finite difference scheme with second order accuracy is obtained for approximately instead of in the form of the original differential equation.With the boundary conditions and initial values, the value of the electromagnetic field in the space of each moment can be obtained with finite difference time domain method step by step.We recruited a health women volunteers and obtained her CT scan data to establish the female pelvic cavity model. After setting different threshold for different tissues and organs, each layer data was segmented to muscles, bones, skins and fat. Due to the uterus and muscles of grey value has no significant difference, we segmented the uterine and tissue fluid form the results of threshold segmentation by human, and then combining with the knowledge of anatomy to remove artifacts. Finally, the data was imported in Mimics software. Through the optimization, we get the real three-dimensional female pelvic model with organs distinguish clearly. Because the ethical relationship, fetal data can’t directly be obtained from the same way. According to the data reported in literatures, we established the model of 13 weeks fetal, combined with the real three-dimensional female pelvic model, and get pregnant women pelvic three-dimensional model.To improve the efficiency of segmentation, we studied the automatic segmentation method, with no supervision multi-scale Gibbs random field segmentation algorithm The algorithm has strong anti-interference ability, good robustness and high efficiency, but still need artificial segmentation to optimize the results, so in the next experiment, we still use more accurate artificial segmentation data to establish three-dimensional model of the human body.Import the three-dimensional pelvic model into SEMCAD software, according to the human electromagnetic parameters provides by international association of applied physics (conductivity o and relative permittivity ε),assignment them to the corresponding organs, we can establish the three-dimensional electromagnetic model corresponding to the real human body model. The electromagnetic parameters are different when frequency is change, so when we do simulation of different field calculation, assignment the electromagnetic parameters of the corresponding frequency to the model, and the three-dimensional human body electromagnetic model under corresponding field intensity can be obtained.Next, we explored when loading non-uniform load (not pregnant women), the homogeneity of MRI radio frequency B1 field impact by different field strengths. The cross-section through the center point of the uterus was selected, remark maximum field strengths value 1, the other field strengths was normalized and then calculate the dB values of the normalized data. You can see from the results, with radio frequency field increased (64Mhz corresponding 1.5 T,128Mhz corresponding 3.0 T,296Mhz corresponding 7.0 T), B1 field homogeneity decline. By the comparison of B1 field distribution with loading non-pregnant women and pregnant women, it can be seen the magnetic field uniformity of the cross section of the uterus center of pregnant women is further reduced. It will affect the quality of magnetic resonance imaging. The three-dimensional electromagnetic model we have established can help to coil design optimize verification.We calculate the local SAR values and average SAR values of not pregnant women at 1.5 T,3.0 T,3.0 T field strength, and the local SAR values and average SAR values of pregnant women at 1.5 T,3.0 T field strength. Results show that the local SAR value of 1.5 T magnetic resonance does not exceed the safety threshold. With the augmentation of the radio frequency, the SAR values also gradually rise. For not pregnant women, when magnetic flux density is 3.0 T, the local SAR maximum of skin beyond the safety threshold value of FDA. When magnetic flux density is up to 7 T, the local SAR maximums of all six organizes are beyond FDA safety threshold. The average SAR values and local SAR values of pregnant women are higher than not pregnant women. At 3.0 T fetal magnetic resonance, skin average SAR value is beyond the safety threshold value, the average of the amniotic fluid SAR are quite close to the safety threshold. The largest rise of the average SAR is in placenta, and the largest rise of the local SAR value is in uterus. The local SAR maximum of uterus and amniotic fluid are very close to the IEC safety threshold. Amniotic fluid parcel fetus, so the local energy hot fetal may cause harmful to fetal development and relevant security need to be concerned.2 Gradient noise protection for fetal MRIEarly research found that if the pregnancy women were exposed to a certain degree of noise, the development in intrauterine and postnatal will be affected by some degree. At this moment, magnetic resonance system noise limit of IEC safety rules is 140 dB. When the acoustic noise of magnetic resonance gradient coil was more than 99 dB (A), it is required to use hearing protection such as earmuffs (IEC60601-2-33 (2010)). The higher field strength, the stronger gradient noise.As the requirement for magnetic resonance imaging and the development of magnetic resonance technology (the thinner layer thickness, higher field strength, etc.), will lead to higher magnetic gradient noise. When magnetic resonance gradient noise over a safe threshold, the auxiliary protective method can be used for adult protection but the protection for fetus have been no specific studies.This article is in view of the acoustic noise issues of fetal magnetic resonance at high field (3.0 T).We give a simple and feasible protection scheme.When the acoustic noise level reach fetus, it is within the scope of the reliable safety threshold for the MR system conform to the requirements of the product noise standards and contribute to the safety protection of fetal magnetic resonance.It is known as a principle that sound can’t travel in vacuum, so we designed a double semi-cylindrical Plexiglas shield. There are 2 cm between the inner and outer layer, and the bottom has a self-closed exhaust port. Through the exhaust port, the air in the cavity can be pumped out with a vacuum pump and form a vacuum space. Human body or other biological need to be measured can be set in the vacuum shield through the entry. If there is gap between the entry and the vacuum shield, use the sound-absorbing cotton for sealing.At first, we test the sound attenuation of the vacuum shield at different location in a quiet room. Put the microphone of decibel meter into the vacuum shield, we measured and record the decibel value. Experimental results show that, when the source of the noise in decibels 80.4dB, noise amplitude attenuation of the vacuum shield is approximately 7dB and the difference between vary locations is no more than ldB. When we tested-450mbar vacuum degree, the vacuum shield was broken due to inability to withstand the atmospheric pressure. In magnetic resonance gradient noise test, we measured the acoustic noise with vacuum degree-400mbar at the middle of the vacuum shield.Different magnetic resonance scan sequence will produce different intensity acoustic noise. We choose some of the commonly used magnetic resonance imaging sequences to implement the corresponding measurement at 3.0 T magnetic resonance. Results show that the noise level varied with different TR, TE, layer thickness, field of view (FOV). Shorter TR and TE, and thinner layer thickness and field of view, will result in more intense noise. Before using the vacuum shield, the sound levels of the noise are all higher than the required safety threshold. With the vacuum shield we designed, the noise levels were attenuated to the range between 84.4 dB (A) and 104.2 dB (A), except T1WI_THRIVE sequence, the gradient noise levels of other sequences had dropped below 99 dB (A). The attenuation effect of the scanning sequence varied with different sequence. The sequence with the highest attenuation amplitude is T2WI_TSE2 sequence (26.9 dB (A)), and the sequence with the least attenuation amplitude is DWI sequence (15.5 dB (A)). Based on the spectrum analysis of acoustic noise, if there is frequency with strong amplitude in the noise spectrum, attenuation effect of the vacuum shield is strong, and if the noise frequency spectrum energy is spread out, even if the whole decibel value is high, the attenuation amplitude is not good enough.Experiments show that the vacuum shield we designed can effectively reduce the gradient noise generated in magnetic resonance imaging. For part of the conventional sequence, the gradient noise can be reduced below the international standard safety threshold and can effectively provide noise protection for fetal magnetic resonance imaging.